Seasonal Transport in Mars’ Mesosphere revealed by Nitric Oxide Nightglow

We analyze the ultraviolet nightglow in the atmosphere of Mars through Nitric Oxide (NO) δ and γ
bands emissions.
On the dayside thermosphere of Mars, solar extreme ultraviolet radiation partly dissociates CO2 and
N2 molecules. O(3P) and N(4S) atoms are carried by the day-to-night hemispheric transport. They
preferentially descend in the nightside mesosphere in the winter hemisphere, where they can
radiatively recombine to form NO(C2Π). The excited molecules promptly relax by emitting photons
in the UV δ bands and in the γ bands through cascades via the A2Σ, v’ = 0 state. These emissions
are thus indicators of the N and O atom fluxes transported from the dayside to Mars’ nightside and
the winter descending circulation pattern from the nightside thermosphere to the mesosphere (e.g.
Bertaux et al., 2005 ; Bougher et al., 1990 ; Cox et al., 2008 ; Gagné et al., 2013 ; Gérard et al.,
2008 ; Stiepen et al., 2015, 2017).
Observations of these emissions have been accumulated on a large dataset of nightside disk images
and vertical profiles obtained at the limb by the Imaging Ultraviolet Spectrograph (IUVS,
McClintock et al., 2015) instrument when the Mars Atmosphere and Volatile EvolutioN (MAVEN)
spacecraft is at its apoapsis and its periapsis phases along its orbit, respectively.
We present discussion on the variability in the brightness and altitude and of the emission with
season, geographical position and local time and possible interpretation for local and global changes
in the mesosphere dynamics. IUVS images and limb scans reveal unexpected complex structure of
the emission. The brightest emission is observed close to the winter pole. The emission is also
surprisingly more intense in some longitude, revealing possible impact of tides and waves in Mars’
nightside mesosphere. Observations also reveal spots and streaks, indicating irregularities in the
wind circulation pattern. The disk images and limb profiles are compared to the LMD-MGCM
model (Gonzàlez-Galindo et al., 2009 ; Lopez-Valverde et al., 2011) to focus on the seasonal, local
time and geographical influences on the NO Nightglow emission. We will also provide a statistical
study of the regions of enhanced brightness (i.e. splotches and streaks) and discuss possible
interpretation from the comparison to the GCM.